The invention to which this application relates is particularly although not necessarily exclusively, to apparatus and a method for the application of a type of coating known as a diamond-like carbon coating to a substrate surface.
Diamond-like carbon coatings are well known and properties of the same such as the hardness, wear resistance and the like are known. The current invention relates to an improvement in the application of a diamond-like carbon coating which further enhances the characteristics of the coating over and above the conventional coatings of this type. A further type of carbon coating is one which has a graphite-like structure which has a lower friction and much better wear resistance than diamond-like carbon coatings which include metal and retain the advantageous properties at high loads in that they have had exceptional load bearing capacity. One such coating is described in the applicant's co-pending application EP-1036208.
There are a number of different known ways of applying diamond-like carbon coatings to a substrate surface, each of which are used commercially with varying degrees of success and for particular applications. Thus particular types of application may be used to suit specific purposes and/or end uses of articles including the coating.
Typically, it is known to be possible to deposit a carbon coating with a diamond-like structure (hereinafter referred to as a diamond-like carbon coating), by creating a glow discharge to a substrate to be coated in a controlled atmosphere containing a hydrocarbon gas. The glow discharge causes a breakdown of the hydrocarbon gas and the carbon from said gas is deposited on the substrate. One well known embodiment of this method is to use an RF discharge on the substrate as described in the Holland and Ojha publication in the Journal, Vacuum 26, page 53 1975.
There are many variations on this method and, by controlling the deposition parameters, the diamond-like carbon coating which is achieved can have variations in characteristics and properties to suit specific uses. Indeed, discharges other than an RF discharge can be used, the power and voltage applied can be varied, differing hydrocarbon gases can be used and/or mixed with other gases such as argon and hydrogen and thus it will be seen that the variation of various parameters of the coating method can be controlled to provide the required end result.
While the application of this type of coating in accordance with this method can be successful in terms of the hardness of the coating, the diamond-like carbon coatings have been found to be relatively brittle and achieving adhesion of the coating to the substrate surface is found to be difficult to achieve to any significant degree.
Conventionally, in order to attempt to overcome these problems, modifications to the deposition procedure and method have been made. One change has been to deposit a small amount of metal along with the carbon from the gas. One such method is described in the document Grischke Bewilogua and Dimigen “Materials and Manufacturing Processes 8(4&5) 407 1993”. The presence of the metal is found to reduce the internal stresses of the coating when applied and, as a result, produces a less brittle coating. However it also reduces the hardness of the coating.
For the application of diamond-like coatings containing metal in a coating chamber, RF power is applied to the substrate to be coated and this serves to break down the hydrocarbon gas in the vicinity of the substrate and so deposit the diamond-like coating on the substrate surface. At the same time, metal is deposited by sputtering from a metal target onto the substrate and hence the coating which is achieved has a diamond-like carbon coating property with metal incorporated therein. However, there are known to be a number of disadvantages to this method in that it is necessary to use a matching network and tuner to reduce the reflected power from the RF power supply and, for large scale deposition systems, the RF power supply, with the associated matching network and tuner, are expensive and have reliability problems as well as convenience problems when compared to the use of DC power supplies.
For example, it is necessary to provide extensive shielding to prevent damaging leakage of the RF power in the chamber and the matching network required has to be able to provide a wide range of impedance values corresponding to the wide range of substrate loads in the coating chamber and so RF power supply deposition methods are conventionally regarded as being inconvenient for a large scale coating system. Furthermore, when RF power is applied to the substrate there is no ability to control, independently, two deposition parameters, namely the voltage applied to the substrates and the ion current drawn by the substrates.
Despite the disadvantages, RF power is typically used, as the diamond-like carbon coatings are electrically insulating. Conventionally, if DC power is to be used then the coating is required to be conducting. This can be achieved by including a higher metal content in the coating but this type of coating has less desirable properties than the RF deposited metal containing Diamond Like Coating (DLC).